In the previous chapter, Russell explored the principle of induction, showing both its necessity for validating arguments and its limitations. The principle can never be proved by experience because the possibility of an alternative may challenge an assumption’s validity. Russell explains that there are other principles that work similarly, including the principle of inference. Humans draw inferences based on a strategy of logic. For example, the knowledge that yesterday was the 15th leads to the logical inference that today is the 16th. Philosophers recognize an important quality in principles like inference and induction: Human knowledge is derived from other places besides direct sensory experience.

Russell outlines three laws of thought that permeate philosophical tradition. First, the law of identity asserts that whatever exists must truly exist. Second, the law of contradiction states that an object cannot both be and not be. Third, the law of excluded middle suggests that an object must be one or the other: existing or not existing. Russell argues that these principles have nothing to do with thought; instead, they are related to priori knowledge.

Russell’s epistemological theory combines both priori knowledge, which suggests that all knowledge is innate, and

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